leetcode+多路归并排序

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//多路归并
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* head = new ListNode(0);
        ListNode* temp = head;
        while(l1 && l2){
            if(l1->val < l2->val){
                head->next = l1;
                head = head->next;
                l1 = l1->next;
            }
            else{
                head->next = l2;
                head = head->next;
                l2 = l2->next;
            }
        }
        if(l1){
            head-> next =l1;
        }
        else{
            head->next = l2;
        }
        return temp->next;
    }

    ListNode* mergeKLists(vector<ListNode*>& lists) {
        int n = lists.size();
        if(n==0) return NULL;
        while (n>1) {
            int k = (n+1) / 2;
            for(int i=0; i<n/2; i++){
                lists[i] = mergeTwoLists(lists[i], lists[i+k]);//归并分开看
            }
            n = k;
        }
        return lists[0];
    }
};